\documentclass{article}
\usepackage[style=authoryear]{biblatex}
\addbibresource{ref1.bib}
\addbibresource{ref2.bib}

\begin{document}

% Warning--entry type for "nostyle" isn't style-file defined
% --line 13 of file ref2.bib
\cite{nostyle}

% Warning--I didn't find a database entry for "undefined"
\cite{undefined}

\printbibliography
\end{document}
